Internal combustion engines (e.g., diesel engines) typically generate a gas flow that contains varying amounts of particulate matter (PM). The amount and size distribution of particulate matter in the gas flow tends to vary with engine operating conditions, such as fuel injection timing, injection pressure, or the engine speed to load relationship. Adjustment of these conditions may be useful in reducing particulate matter emissions and particulate matter sizes from the engine. Reducing particulate matter emissions from internal combustion engines is environmentally favorable. In addition, particulate matter measurements for diesel gas is useful for on-board (e.g., mounted on a vehicle) diagnostics of PM filters and reduction of emissions through combustion control.
Conventional technologies that can be used for on-board monitoring of particulate matter in gas flows include wire and ceramic-based sensors. Both types of sensors apply a high voltage to one of two electrodes and measure the current or charge on the other electrode. The electrode measurement is correlated with a specific particulate matter concentration. Wire sensors use conductive wires as the electrodes. Ceramic sensors use conductive traces, which are disposed on ceramic substrates or structures, as the electrodes. Some ceramic sensors are superior to wire sensors at least because they are easier to manufacture, cost less than wire sensors, may have more vibration resistance, and are more robust in adverse operating environments. By way of comparison, ceramic-based electrodes are more rigid than wire electrodes and, hence, vibrate less, maintain a more consistent distance between the electrodes, and produce less noise in the resulting electrical signal. However, both wire and ceramic sensors are subject to de-calibration and baseline drift of the sensor due to accumulation of soot (i.e., particulate matter) on and between the electrodes. Additionally, conventional wire sensors have a limited area where the electrodes face each other, so the resulting sensor signals may be relatively small.
For ceramic sensors, electrode heaters can be integrated into the ceramic sensor structure to burn off soot at the electrodes. However, the electrode heaters can significantly increase the temperature of the electrodes and the electrical leads connecting the electrodes to the electronics module. Increasing the temperature of the electrodes at the same time that a high voltage is applied to the electrodes (e.g., one of the electrodes) can result in electrical leakage through the ceramic materials because the increased temperature decreases the insulating properties of the ceramic materials. This type of electrical leakage can impair the accuracy of the sensor measurements.